A new strain of the influenza virus had emerged in the spring, an aggressively infectious illness that triggered sporadic outbreaks throughout the Northern Hemisphere's summer, a normally flu-free season. As scientists raced to develop and produce a vaccine, Americans debated how seriously to take the threat of a pandemic.

During the warm months, the illness struck mostly in group-living situations such as summer camps and military bases. But the virus was spreading inexorably, and the fall's mass gathering of children in the new school year would be like wind blowing on dry grass that had been showered with sparks. By October, the country was afire with influenza.

As public health experts prepare for a big outbreak of H1N1 influenza in the United States, they are looking to the past as they try to anticipate what may happen in the next two or three months.

The pandemic virus of 1957 caused an illness that was briefly harsh but rarely fatal. Like this year's strain, it emerged in the spring and smoldered over the summer, its traditionally quiet season. More than 80 summer camps have suffered flu outbreaks in the past two months; in 1957 the picture was much the same. As with many pandemic strains, the Asian flu also had an inordinate effect on younger people. By the time the 1957 pandemic was over, 40 percent of deaths had occurred among people younger than 65, a much larger fraction than in outbreaks of seasonal flu. Analysis of the first 44,000 American cases of this year's pandemic virus revealed the infection was most common in people age 5 to 24 -- 20 times as common, in fact, as in people older than 65. Eighty-three percent of fatal cases occurred in people younger than 65.

Such similarities suggest the pandemic of 1957 may be a particularly instructive model for what's to come.

The Attack Begins

The origins of the flu outbreak reported in Hong Kong in April 1957 remain shrouded in mystery. What was soon known, however, was that it had an unusually high "attack rate": A substantial fraction of people fell ill once the virus began to circulate in a community. That prompted U.S. officials to ask for samples of the virus, and the first arrived in Washington on May 13. Scientists soon determined it was a radically new strain of the H2N2 class of influenza.

The new bug hopscotched its way into North America, striking first on naval vessels and coastal military installations. Flu appeared in Newport, R.I., on June 2 and soon after in California. The attack rate was 30 percent on the ships and 5 to 10 percent on shore stations, according to the reports from the Communicable Disease Center (now the Centers for Disease Control and Prevention).

Soon, the virus moved into the civilian world.

The first well-studied outbreak occurred in Grinnell, Iowa, where 1,688 people from 43 states and 9 countries, most of them women, had gathered in June for a church conference. Among them were 200 Californians, including at least one from Davis, where an outbreak had recently occurred.

The first Iowa case appeared on June 26. Within a few days there were dozens more, and a dormitory was designated an infirmary. By July 1, with the toll at 200 cases, the conference disbanded.

Returning home, the delegates effectively seeded the entire continent with the new virus. Curiously, no community-wide outbreaks followed. That was explained by saying summer's heat and humidity are somewhat inimical to flu -- a belief largely borne out by subsequent experiments. Whatever the reason, it helped assure people at the CDC there wasn't going to be an all-out epidemic . . . at least until fall.